Hi Kaspanova,
The BT Infinity product has recently been updated to offer up to 80/20 Mb service, many of the other BT Wholesale providers are offering the original unto 40/10Mb service. So I would suggest the BT estimate is accurate whereas Sky are just quoting the maximum speed they can offer and would not trust this figure.
Normally you pay a premium rental if you want the higher upload speed as previously the residential products were up to 2Mb on upload speed which is normally fine for your average user.
I use and recommend Plusnet FTTC, I've had it around 9 months and think it is great. My average DL speed is 37.5Mb and UL is 1.7Mb. I pay £21.50 per month for a 120GB usage cap and the nearest i have got to this in any particular month is 65GB.
Monthly cost with Plusnet for this is £21.50 per month and you get a discount for each referral you give that is applied each month for as long as your referral remains a customer. Depending on the product your referral takes this can be a further reduction of 25 to 75p per month for each referral. If you do go Plusnet feel free to use my username, BigAl217, as someone who referred you
I don't think you can get the premium upload on Plusnet so if you need this it won't be for you. Plusnet are also owned by BT and a way they provide there FTTC product at a low price but without the BT name attached to it, a bit like GiffGaff Mobile which is owned by o2 IIRC.
Anyway good luck I haven't looked back since changing from my creaky 4.5Mb dl ADSL2+ service.
Alan